The present invention relates to the field of memory cards of the contactless type. Such cards are commonly used, e.g. to constitute access badges or remote payment cards.
Contactless memory cards generally comprise an integrated circuit and an associated coupling member, both embedded in a flat insulating assembly. The particular type of contactless memory card to which the invention applies is represented by cards having a card body of plastics material with an integrated circuit embedded in one face thereof, and with a coupling antenna on said face which is connected to two contact pads of the integrated circuit, and an insulating protective sheet covering said face of the card body.
In practice, the antenna is organized in a spiral pattern with the two ends of the spiral being connected to respective contact pads of the integrated circuit. The inner end of the spiral pattern can easily be connected to one of the two contact pads. However for the outer end it is necessary to provide a connection link which passes over the turns forming the antenna. That organization is made necessary by the need for the conductive track which forms the antenna to be as wide as possible, with the purpose of having a very wide track being associated with the need for resistance that is as low as possible so as to have an antenna with a Q factor that is as high as possible (where the Q or xe2x80x9cqualityxe2x80x9d factor corresponds to the ratio of impedance over resistance). The technique of making antennas by using silkscreen printing to deposit a conductive paste is well known and well mastered. However, such pastes have high resistivity, thereby giving antennas whose (pure) resistance is high, reaching several tens of ohms. That is why more and more use is being made of antennas that are made of very wide silkscreen printed tracks, of maximum section, so as to reduce the resistance thereof. Obtaining a connection to the integrated circuit via conductive links situated at two superposed levels also requires manufacturers both to provide both a conductive bridge connecting the outer end of the antenna to one of the contact pads of the integrated circuit, and insulation disposed on the corresponding zone of the turns so that the conductive bridge can be placed over the insulation so as to avoid making contact with the turns over which it passes. Although silkscreen printing makes it possible to build up these various levels in succession at high rates of throughput, it nevertheless remains that three steps are required, and that the necessary accuracy is high, thereby further increasing manufacturing cost.
Document EP-A-0 737 935 discloses a contactless card having an internal layer carrying a spiral antenna and an integrated circuit placed across the turns of the antenna. The outer and inner ends of the antenna are connected to respective pads by means of an insulating layer extending between the turns and facing the integrated circuit, and made locally conductive in register with each of the pads.
Finally, reference can be made to document EP-A-0 671 705 which describes a dual-connection type card having a spiral antenna and an integrated circuit disposed over the turns of the antenna, which circuit is connected to the ends of the antenna via associated connection elements.
Cards made in accordance with either of the two above-specified documents are complex in structure, thereby giving rise to manufacturing costs that are high.
A particular object of the invention is to solve that problem, by designing a different organization enabling manufacture to be simplified and reducing production costs, without significant loss of Q factor compared with contactless cards of the above-mentioned type.
The invention thus seeks to provide a memory card whose structure makes it possible to minimize manufacturing steps, so as to have a manufacturing cost that is highly competitive while retaining a Q factor that is comparable to that of existing memory cards in which the antenna is made in three successive silkscreen printing steps.
According to the invention, this problem is solved by a memory card of the contactless type, comprising a card body of plastics material having an integrated circuit embedded in one face thereof, with a coupling antenna on said face being connected to two contact pads of the integrated circuit, and with an insulating protective sheet covering said face of the card body, wherein the antenna is organized as a spiral pattern whose turns pass over the flush face of the integrated circuit.
Such an organization for the antenna makes it possible to have a continuous profile passing over the flush face of the integrated circuit. The card made in this way is simple in structure so it is of relatively low cost to manufacture.
Advantageously, the antenna is constituted by a conductive track whose resistance is locally increased in a zone of the face of the card body directly surrounding the integrated circuit to connection with very narrow conductive lines organized on the flush face of the integrated circuit and passing between the two contact pads of said integrated circuit.
The narrowness of these lines makes them easier to implement on the flush face of the integrated circuit between the two contact pads. In addition, the local increase in resistance gives rise to only a limited increase in the resistance of the antenna, thus making it possible to conserve a high Q factor.
Preferably, the antenna-forming conductive track presents, in the above-specified zone, and for each of its turns, a width that is locally reduced, thereby locally increasing its resistance, and in particular a width that decreases progressively both for connection to the contact pads of the integrated circuit and for connection to the conductive lines passing between said contact pads. The locally narrowed width in a predetermined zone directly surrounding the integrated circuit serves specifically to connect with very narrow conductive lines. This zone is selected to be as small as possible so that the increase in resistance that results from the local reduction in section is as insignificant as possible compared with the resistance that would be offered by a traditional antenna of the same length.
In a particular embodiment, the width of the conductive track can be as great as 3 mm outside the above-specified zone, and can decrease to about 0.2 mm inside said zone close to the conductive lines organized on the flush face of the integrated circuit. The length of the segments of the conductive track having increased resistance constitutes no more than 10% of the total length of said conductive track.
The conductive track may be made completely out of conductive polymer deposited by silkscreen printing. In which case, a single manufacturing step suffices to make the entire antenna directly.
In a first variant, provision can be made for the flush face of the integrated circuit to present a pattern of conductive lines with connection lands at their ends, all of said lines and lands being made by metallization, and for the remaining portion of the conductive track to be made of conductive polymer deposited by silkscreen printing on the face of the card body. In which case, two successive manufacturing steps need to be implemented, but considerably lower resistivity is obtained because of the metallization, and the metallization step can be implemented on the silicon wafers themselves prior to being cut up to provide the integrated circuits. This makes it possible to metallize several tens of thousands of integrated circuits on a single occasion.
In another variant, it is possible to provide for the portion of the conductive track outside the above-mentioned zone to be made of metal deposited on the face of the card body, for the flush face of the integrated circuit to present a pattern of conductive lines with connection lands at their ends, all of said lines and lands being made by metallization, and for the remaining portion of the conductive track which is inside said above-mentioned zone to be made of conductive paste deposited by silkscreen printing on the face of the card body. In which case, even though it is necessary to implement three successive manufacturing steps, the accuracy required is much less than that with prior implementations, such that the machines used are of lower cost.